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VIETNAM STANDARD TCVN 11080:2015

VIETNAM STANDARD  TCVN 11080:2015

This standard specifies a method for the determination of the hydroxyl number in animal and vegetable fats and oils and their products, such as fatty alcohols, mono-glycerides, diglycerides, and hydroxystearic acids.

VIETNAM STANDARD TCVN 11080:2015

Animal and vegetable fats and oils - Determination of hydroxyl value by acetylation method

1. Scope of application

This standard specifies a method for the determination of the hydroxyl number in animal and vegetable fats and oils and their products, such as fatty alcohols, mono-glycerides, diglycerides, and hydroxystearic acids.

2. References

The following references are essential for the application of this standard. For dated references, the version stated applies. For undated referenced documents, the latest version, including amendments and supplements (if any), applies.

ISO 661, Animal and vegetable fats and oils — Preparation of test samples.

3. Terms and definitions

hydroxyl value (hydroxyl value): is the number of milligrams of potassium hydroxide equivalent to the amount of hydroxyl in 1 g of fat.

4. Principles

Carry out acetylation of the free hydroxyl groups in the sample with pyridine-acetic anhydride solution. The excess acetic anhydride was converted to acetic acid and titrated with an alcohol solution of potassium hydroxide.

5. Reagents

Use only reagents of analytical grade. Water used is distilled water or water of equivalent purity, unless otherwise specified.

5.1. Pyridin

Re-distilled at 114 °C to 115 °C.

5.2  Acetic anhydride

Freshly prepared.

5.3 Pyridin-acetic anhydride solution

Mix 3 parts by volume of pyridine (5.1) with 1 part by volume of acetic anhydride (5.2). Prepare the solution immediately before use.

5.4 n-butyl alcohol

Neutralize the solution with 0.5 M potassium hydroxide solution so that when the phenolphthalein indicator is added, the titration endpoint is pale pink.

5.5 Potassium hydroxide in alcohol standard solution 0.5 M

NOTE The concentration of the solution should be ≥ 0,5 M so that during titration the blank does not need to be refilled if a 50 ml burette is used.

5.6  phenolphthalein indicator.

6. Equipment, tools

Using common laboratory equipment and tools and the following specific equipment and tools:

 

6.1  Conical flask, capacity 250 ml, roughneck, fitting.

6.2   Analytical balance, capable of weighing to the nearest 0.001 g.

6.3  Water bath, capable of being maintained between 114 °C and 115°C.

6.4  Reflux condenser.

6.5 Pipet

Sample preparation
Samples are prepared according to the method described in ISO 661.

How to proceed?
Based on the expected hydroxyl value, weigh an appropriate amount of the sample, to the nearest 0,001 g, into the conical flask (6.1), for acetylation. Suitable sample weights are given in Table 1.

Table 1 - Mass of test portion for acetylation according to expected hydroxyl number

Expected hydroxyl number

Weight (g)

0 to 20

10 ± 0,1

20 to 50

5

50 to 100

3

100 to 200

2

 

Weigh approximately 9,0 g to 11,0 g of the test portion into another conical flask (6.1) to determine the acidity (for fatty acids such as hydroxystearic acid, take 0,9 g to 1,1 g of the test portion). 

Pipette (6.5) 5,0 ml of pyridine acetic anhydride solution (5.3) into the sample container prepared for acetylation. For samples with an expected hydroxyl number from 0 to 20, add 5 ml of pyridine (5.1). Mix thoroughly by gently swirling the flask.

To carry out the blank test, pipette (6.5) 5 ml of pyridine acetic anhydride solution (5.3) into another conical flask (and 5 ml of pyridine, if pyridine was used on the test portion for acetylation).

Place the flasks in the water bath (6.3) under the reflux condenser (6.4) and heat for 1 h. Do not use electric stoves or burners.

Add 10 ml of water through the reflux condenser and heat for a further 10 min. Allow the flask with the condenser to cool, add 25 ml of n-butyl alcohol (5.4) to about half, through the condenser, remove the reflux condenser and use the remainder to wash the walls of the flask. Add 1 ml of phenolphthalein (5.6) and titrate with a standard solution of potassium hydroxide in 0.5 M alcohol (5.5) to a pale pink titration endpoint.

7. Calculate and represent the result

The hydroxyl number, HV, is calculated according to the following formula:

In there:

V:  is the volume of potassium hydroxide solution used for titration, in milliliters (ml);

B:  is the volume of potassium hydroxide solution used for the blank titration, in milliliters (ml);

W: is the mass of the sample used to carry out the acetylation, in grams (g).

C: is the mass of the sample used for the acid titration, in grams (g);

S:  is the volume of potassium hydroxide solution used for the acetylated sample, in milliliters (ml);

c : is the molar concentration of the potassium hydroxide solution, in moles per liter (mol/l);

56.1:  is the molecular mass of potassium hydroxide, in grams per mole (g/mol)

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